1. Field of the Invention
The present invention relates to a locating method and a device for carrying it out and, in particular, an efficient control of at least one accurate locating method and at least one energy-saving locating method on a radio-based terminal (e.g. mobile phone) for carrying out proactive, location-based services.
2. Description of the Related Art
Location-based services (LBS's) provide users with information and services according to their whereabouts. While the user is provided with location-based information on request in the case of reactive LBS's, proactive LBS's automatically initiate service actions when predefined spatial events occur, such as when a user is approaching a point of interest (PoI).
While various reactive LBS's are already available, proactive LBS's have until now only been available on a small scale and in a very rudimentary form.
Examples of proactive LBS's are the following services:                Proactive tourist guide: Users are automatically supplied with relevant background information, as soon as they approach a particular landmark.        Child tracker: A child is fitted with a mobile terminal, which informs its parents as soon as it leaves a predetermined zone (e.g. the school playground).        Buddy tracker: Two mobile subscribers are notified when they are close to one another (e.g. in the context of a community service).        
The main problem with proactive LBS's until now has been executing them efficiently. This relates in particular to the basic mechanism that affects all proactive LBS's, namely, the automatic detection of whether and when the user either enters or leaves a previously defined geographical zone, hereinafter referred to as the update zone. This should happen with the greatest possible temporal and spatial resolution.
For example, the update zone may be defined as a circle with a predetermined centre and radius or as a polygon.
Various locating methods are conceivable when it comes to achieving a sufficiently accurate spatial resolution, for example, terminal-based methods such as GPS, assisted GPS, the future European satellite system Galileo or the Enhanced Observed Time Difference (E-OTD) method, which is part of the GSM standard. With this method, the user's mobile radio terminal determines its position itself.
Alternatively, network-based locating systems such as Uplink Time Difference of Arrival (U-TDoA) are known, in which the position of the terminal within the network is determined based on radio signals received from the terminal.
Other known locating methods include WLAN and GSM fingerprinting, which can run in network-supported or terminal-based mode, in which the terminal takes signal strength measurements from the surrounding base stations and constantly transmits the measurements to a network server, so that the position can be determined.
The known locating methods have the following disadvantage in relation to the identification of update zones. In order to achieve the high temporal resolution required, the locating method must be operated almost continuously. In some cases this results in high (technical) costs. In the case of terminal-based methods, the continuous position calculation leads to high power consumption by the terminal. In the case of network-based and network-supported methods, signals must again be transmitted constantly from the terminal, which puts a strain on the limited air interface and leads to significant scalability problems. Moreover, the frequent transmissions likewise lead to high energy consumption.